In this art it is useful to be conscious of the difference between "sponge iron" and "iron sponge", as these terms are commonly recognized by those skilled in this field. The more commonly known term is "sponge iron", which by more modern and precise terminology (in order to avoid confusion in these terms) is preferably called DRI, or direct reduced iron. Typically, sponge iron may have a metalization of 80% to 95%. Since sponge iron is made by removing oxygen at reducing temperatures below the melting point of the iron-containing material from which it is made, the resulting sponge iron produce (DRI) is typically quite porous with high specific surface.
In contrast, iron oxides, in the form of ore or otherwise, are typically not very porous. Thus to take advantage of the known ability of iron oxide to remove sulfur from sulfur-containing gases, the prior art has developed a porous form of ferric oxide known as "iron sponge". This is attained by creating "mixed oxides" (where finely divided iron oxide is supported on materials of large surface and light weight, typically, wood shavings, wood fluff, or wood chips). Such mixed oxides are also referred to as "iron sponge".
In view of the foregoing, "sponge iron" when used herein, is intended to mean "DRI" (direct reduced iron).
The desulfurization of sulfur-containing gas streams such as sour natural gas is an important expedient in providing commercially viable and valuable sources of process gas or fuel suitable for a multitude of uses. While natural gas is found in many regions of the world, many of the available sources of natural gas frequently additionally contain sulfur contaminants like hydrogen sulfide, carbonyl sulfide and mercaptans.
The term "natural gas" is used herein in its usual meaning as would normally be understood by ones of ordinary skill in the art; as follows: natural gas is a mixture of low molecular weight paraffin series hydrocarbons (methane, ethane, propane, and butane), possibly including with progressively smaller amounts of higher hydrocarbons, and sometimes containing some nitrogen, carbon dioxide, sulfur contaminants (such as hydrogen sulfide, H.sub.2 S; carbonyl sulfide, COS; and mercaptans), and/or helium; other components, such as hydrogen, would be present, if at all, in only trace amounts (less than 1.0%). Methane usually is the predominant constituent of natural gas, typically, 85% or more.
Depending upon the particular commercial process or end use, the sulfur-containing natural gas stream must be suitably desulfurized to remove substantially all sulfur compounds present therein. Frequently, it is necessary to lower the sulfur concentration to low levels in the range of 0.1 to 0.4 ppm. For example, in processes requiring the use of a catalytic reformer for conversion of a natural gas feed to a reducing gas effective for the reduction of iron ore to sponge iron or for the production of hydrogen, methanol, ammonia, and for other uses, the sulfur content of the natural gas should be less than about 0.2 ppm to avoid sulfur poisoning of the reformer catalyst.
It is noted that the adsorption of sulfur compounds contained in hydrocarbon gas streams by contact with metals or metallic compounds is generally known. Exemplary is U.S. Pat. No. 2,551,905 directed to a process for the desulfurization of a hydrocarbon gas by the countercurrent contact of a sour gas stream with adsorbent ceramic and metal-oxide pellets such as iron oxide at elevated temperatures. In U.S. Pat. No. 3,199,946 a method is disclosed for the removal of hydrogen sulfide from hydrocarbon fuel gases using adsorbent compositions including finely-divided iron metal, moisture and a water soluble alkali metal carbonate, bicarbonate or hydroxide. Further, in U.S. Pat. No. 3,151,973 a method for the production of low sulfur molten iron is disclosed by passing a reducing gas stream through a bed of sponge iron to absorb the sulfur contaminants. Finally, U.S. Pat. No. 3,816,101 discloses the removal of hydrogen sulfide from a stream of process gas at low temperatures.
While the prior art generally recognizes a variety of methods directed to the desulfurization of carbonaceous gases including the use of molecular sieves and metallic compounds such as zinc oxide or even sponge iron, a real need continues to exist for a more economical, efficient and effective method for the desulfurization of natural gas. That is, a method by which a cost-effective, efficient and readily available system can be used to carry out the desulfurization reaction at an acceptable rate of reaction without cracking of components of the natural gas.